As they hunt for revenue to pay for their sprawling spending bill and try to unite a fractured caucus, Democrats are attempting to rewrite the United States tax code in a matter of days, proposing the kind of sweeping changes to how America taxes businesses and individuals that would normally take months or years to enact…

The frantic attempt to overhaul the complex U.S. tax code remained in a state of flux on Wednesday, with Senator Joe Manchin III and some House Democrats expressing reservations about a tax on billionaires that was proposed earlier in the day by Senator Ron Wyden of Oregon. On Tuesday, Mr. Manchin shot down a plan that would have given the Internal Revenue Service more visibility into certain taxpayers’ bank accounts in order to catch tax cheats, forcing a group of Senate Democrats who support the provision to try to negotiate a compromise. Mr. Manchin’s opposition to a new federal paid leave program also appeared to doom its chances of being included in the final legislation, although supporters of the provision said they would fight to keep it intact.

Senator Mark Warner, a Virginia Democrat, acknowledged on Wednesday that the rapid pace of the legislative process posed risks and said it would be preferable to “allow some of this very, very complicated tax policy to get an appropriate airing back and forth.”